The present invention relates to a differential amplifier which has high input impedance and a very small DC offset voltage.
FIG. 1 shows a conventional differential amplifier. This differential amplifier has pnp transistors TR1 and TR2 having bases connected to inverting and non-inverting input terminals IIT and NIT, a current mirror circuit of npn transistors TR3 and TR4 for transmitting the same current to these transistors TR1 and TR2, an npn output transistor TR5 having the base coupled to the collector of the transistor TR4, a constant current source CS1 for supplying a current to the transistors TR1 and TR2, and a constant current source CS2 for supplying a current to the transistor TR5.
Bipolar monolithic integrated circuits are generally manufactured by a process which is suitable for mass-production. For this reason, pnp transistors are often formed as lateral pnp transistors. However, pnp transistors have a low current amplification factor and this renders circuit design difficult.
The differential amplifier using the pnp transistors TR1 and TR2 in FIG. 1 can be operated at a low voltage. However, if the current amplification factor of the pnp transistors TR1 and TR2 is small, the base currents of these pnp transistors TR1 and TR2 are no longer negligible. Thus, in order to realize a differential amplifier having a high input impedance and a small DC offset voltage, the emitter currents of the pnp transistors TR1 and TR2 must be kept to a minimum. Therefore, the output current of the constant current source CS1 must be kept small. This requires a circuit for obtaining a small constant current and renders the construction of the differential amplifier complex.
The differential amplifier shown in FIG. 2 is the same as that shown in FIG. 1 except that the pnp transistor TR1 is replaced with a transistor circuit of pnp transistors TR1A and TR1B, and the pnp transistor TR2 is replaced with a transistor circuit of transistors TR2A and TR2B. When these transistor circuits are used, their base currents can be kept at negligible values and a differential amplifier of high input impedance can be realized. However, in this case, since the equivalent base-emitter voltage of the transistor circuits becomes twice that of the pnp transistor TR1 or TR2, the differential amplifier shown in FIG. 2 cannot operate at a very low voltage, for example, 0.9 V.